Excimer laser patterning of a novel resist

ABSTRACT

A dual layer resist configuration is employed for photopatterning high resolution conductive patterns on underlying polymeric or ceramic substrates, particularly substrates exhibiting surface roughness and non-planar design features such as channels, bosses and ridges. More particularly, a thin underlayer of ablatable photoabsorptive polymer is disposed on a metal coated substrate, after which a thicker layer of substantially transparent material is disposed over the polymer. A beam of laser energy, such as that produced by an ultraviolet excimer laser, is directed through the upper layer and is absorbed by the lower layer which is ablated and simultaneously removes the thick layer above it. This results in the ability to etch high resolution features on polymeric and other substrates, particularly copper coated polyetherimide circuit boards. The resist system is also applicable to VLSI wafers even though such wafers usually do not exhibit surface roughness on the scale generally considered herein. It is also equally applicable in various high density interconnect systems used for the direct connection of chip devices. A mask for patterning and a method for making it are also seen to be desirable because of the high laser energy densities generally desired for thorough ablation.

BACKGROUND OF THE INVENTION

The present invention is generally directed to a dual layer photoresistsystem which is particularly applicable to the formation of highresolution conductive patterns on insulative substrates exhibitingsurface roughness and/or non-planar surface variations. Moreparticularly, the present invention is directed to a two level resistconfiguration and method for photopatterning which employs a thin layerof ablatable photoabsorptive polymer over which is disposed a thickerlayer of substantially transparent material which is exploded awayduring laser ablation of the underlying photoabsorptive layer.

It is nearly impossible to photopattern surfaces exhibiting surfaceirregularities and/or non-planar aspects, by normal resist methods. Somesurface irregularities may occur as a result of the desire to employfiller material such as glass fibers for added strength. Other surfaceirregularities may occur as a direct result of structural designfeatures incorporated into a molded part. Such features includechannels, ridges and bosses, for example. For such workpieces, the useof resist laminates, spin coatings and the like prevent proper resistthickness control and contact maintenance so as to render such resistsand processes non-viable.

Moreover, parts patterned using conventional photoresists such as thoseknown in the semiconductor patterning arts are generally fine tuned foruse with medium to high pressure mercury arc lamps and do not performwell, if at all, in the presence of collimated, single frequencycomponent radiation such as that produced by lasers, especiallyultraviolet lasers. With regard to these conventional resists, a problemexists in that the laser energy is absorbed in the top layer, and oncecured, this layer then blinds the surface beneath it, thus rendering thecuring of this subsurface material extremely difficult, if notimpossible. The resist system and process of the present invention solvethese problems and others as is more particularly described below.

In particular, the resist configuration and method of the presentinvention is directly applicable to the patterning of three-dimensionalelectronic circuit boards, modules and the like. Additionally, thepresent invention is also applicable to patterning three dimensionalcircuit boards and/or surfaces which also include feed-throughapertures. As used herein, and in the appended claims, the use of thephrase "three-dimensional" refers to surfaces which are rough, eitherbecause of a molding process or because of the use of filler material.This phrase also refers herein to surfaces which exhibit structuraldesign features such as channels, ridges, bosses and the like.

The concept of patterning such three-dimensional boards with a mask withthe use of normal light sources is almost impossible. The 1° to 3°diffractions in the light sources, plus lens irregularities makenon-contact mask use extremely difficult due to the undercutting of thepatterns. Contact mask processes are virtually impossible due to thethree-dimensional nature of the surface. Pattern compensations can aidin this problem but edge definition is nonetheless reduced. Furthermore,the use of a standard light source with its associated long exposuretime over large areas is prohibitive. For example, it takes minutes toexpose large areas with a fixed 500 watt or 1 kilowatt light source.

Points, tips, bosses, etc. of three-dimensional boards cannot easily becoated with resist material in any way. Even sprayed on resists do notwork well for this application. Furthermore, films of resist, such asRISTON™, cannot be roll laminated to such three-dimensional boards.Additionally, there is no easily procurable film resist having athickness less than approximately 0.7 mils (that is, about 18 microns).In addition, there is none that lasers can expose correctly. In most ofthe cases of interest herein, a resist thickness of 2 to 3 mils (50 to75 microns) is needed to cover the surface finish alone. No normalresist is known which can work at these thicknesses with a device suchas an excimer laser, for example. Furthermore, the concept of disposinga thin resist which is supported by a clear continuous carrier, such asRISTON™, which is removed before developing to give the appearance of athick resist also does not work since the resist layer is confused withthe over-layer due to their mutual solubility with respect to oneanother.

As indicated above, a further area of concern is that the percentage offiller in a polymer substrate, such as one comprising a polyetherimidesuch as ULTEM™, makes coating difficult. This aspect plus variationsresulting from internal mold finish, make product surface finishvariable. Various "pretreats" are, however, employed to promoteadhesion. All of the fillers nonetheless produce a very rough surface.Normal resists cannot coat such surfaces, since resists arenon-conformable to points, edges, etc. yielding voided areas.

In order to achieve fine line and spacing, one needs a well collimatedlight source, a good mask and a good, thin resist. There is a directrelationship between the aspect ratio associated with resist thicknessand the ability to resolve line and area details. More particularly, thethinner the resist, the greater the ability to resolve fine lines.However, the thicker the resist, the more accurate and precise theradiation source must be. Moreover, the radiation source must exhibit anareal energy density sufficiently high to effect the desired changes inthe resist (here ablation). The frequency of the radiation source mustalso be such that the resist is absorptive at that frequency.Additionally, the mask employed should be able to withstand laserradiation bombardment at the required frequencies without degration inits structure or pattern.

Photoresists that are conventionally used in the semiconductor arts areso absorptive at excimer laser frequencies and energies, that properexposure is not feasible. Additionally, positive resists are desiredwhenever through-holes are present. More particularly, if negativeresist material dries in the through-holes, such holes cannot be exposedwith the laser. This is because the holes are typically approximately0.1 inches (100 mils) deep and the absorption is 99.99% in the firstmicron (0.04 mils). In processing, the resist comes out and allows theplated through holes to be destroyed in the etching step. However,positive resists are so-called chain break resists and do not exhibitthis problem.

All of the aforementioned problems have made it nearly impossible tomake production level three-dimensional parts possessing fine lines andfeed-through apertures.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention, atwo level resist configuration comprises a thin underlayer ofphotoabsorptive ablatable, polymer or resist over which is disposed athick upper layer of substantially transparent material. If theconductive material to be patterned is disposed on a substratecomprising a polyetherimide, then the underlayer preferably comprisesmaterial selected from the group consisting of polysulfones. If theunderlying supportive substrate comprises a relatively inert materialsuch as a ceramic, then the choice of underlayer material issignificantly less restrained and may include polyimides, polyamides andpolycarbonates. The upper layer preferably comprises material selectedfrom the group consisting of UV transparent polymers such aspolycarbonates, polymethylmethacrylate (PMMA) and polyethylmethacrylate(PEMA) and similar ultraviolet transparent material. Moreover, inpreferred embodiments of the present invention, the upper and lowerlayers exhibit different solubilities with respect to one another sothat the upper layer and lower layer are not confused, that is, so thatthere is a definite and uniform boundary between the upper and lowerlayers; this enhances the ability of the resist system to form highresolution line patterns. While the resist system of the presentinvention is particularly applicable to situations involving thepatterning of conductive patterns, especially copper patterns, onsubstrates comprising a polyetherimide such as ULTEM™ polymeric resin,it is also applicable to other substrates and conductive materials.However, it is also necessary that material employed to strip the layersdoes not significantly adversely affect the underlying substrate. It is,however, necessary that the upper and lower layer comprise differentmaterial or at least similar materials treated to exhibit differentphotoabsorptive properties so that the lower layer may be ablated andthe upper layer simply blown away by the explosive effects of ablation.

Correspondingly, a method for photopatterning conductive patternspreferably comprises the steps of disposing a thin layer of ablatablephotoabsorptive polymer on a substrate having a conductive layer thereonso that the polymer material is in contact with the conductive layer.Then a thick layer of substantially transparent material is disposedover the thin underlying polymer. Naturally, photoabsorptivity andtransparency are determined with respect to the same frequency range.Then, a beam of incident laser energy is directed through the thicklayer so as to ablate the resist layer and so as to simultaneouslyremove the thick layer directly above the ablated resist layer beneathit. The exposed copper or conductive metal pattern may then be etched.Alternatively, the exposed conductor may be made thicker byelectrodeposition methods, after which the polymer and the thickoverlayer are removed and the resultant workpiece etched so as to leaveconductive material only where it had been built up. In this situation,the use of the upper layer promotes the formation of thick conductivepatterns exhibiting highly vertical side-walls and an excellent aspectratio.

It is noted that a thin layer of ablatable resist is not employable byitself to achieve the desired results. In particular, in thethree-dimensional patterning problem, the resist alone could not acteffectively as a mask for the conductive layer beneath it without beingso thick that soot and debris from its ablation become prohibitive andtend to defeat the process because soot and debris would tend to fallback onto already exposed conductive surfaces and would tend to coat themask and ultimately alter its characteristic patterning.

It is also particularly noted that the process of the present inventionis preferably carried out using an excimer laser operating in theultraviolet range and that the thick upper layer is generallyapproximately 10 times the thickness of the ablatable underlayer. It isalso noted that the present invention is preferably practiced using anon-contact mask arrangement. A contact mask arrangement is, however,not generally employable for the present purposes because of debrisremoval. In the particular case of three-dimensional substrates whichinclude ridges and the like, a contact mask system is virtuallyimpossible and certainly impractical. It is further noted that thepresent invention is preferably carried out in conjunction with a vacuumdebris removal system operating in the vicinity of the ablation site toprevent soot and debris scatter.

Accordingly, it is an object of the present invention to provide aresist and patterning method that enables the manufacture ofthree-dimensional printed circuit boards from molded or otherwisefabricated parts by means of a highly collimated light source such as anexcimer laser.

Another object of the present invention is to provide a dual levelresist system that enables the direct patterning of rough surface boardswith excimer laser light while still forming fine resolution patterns onthe board.

An additional object of the present invention is the formation of a twolevel resist system useful in the manufacture of other resist relatedproducts such as semi-conductor wafers and the decaling of metal partsand the like.

It is a further object of the present invention to produce highresolution conductive patterns on a supportive substrate through thechoice of underlayer and overlayer materials which are compatible inthat the disposition of the one on the other does not cause theformation of confused layers.

It is yet another object of the present invention to immediately patternwithout the necessity of development, this being accomplished by thechoice of underlayer and overlayer material which is removable bysolvents which do not attack the supportive substrate on which theconductive material to be patterned is disposed.

It is a still further object of the present invention to produce aresist system that has a very high latitude for thickness variation,drying, and process conditions and yet is still associated with ease ofremoval without dissolving or damaging an underlying supportivesubstrate.

It is also an object of the present invention to provide removal of ash,soot and debris from the ablation process.

Lastly, but not limited hereto, it is also an object of the presentinvention to provide a resist system that requires minimum exposure froman incident laser beam, this latter being accomplished due to thethinness of the underlayer which absorbs the beam energy in comparisonto the second, transparent layer which masks and shields and isremovable without adversely affecting a supportive substrate.

DESCRIPTION OF THE FIGURES

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. The invention, however, both as to organization andmethod of practice, together with further objects and advantagesthereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings in which:

FIG. 1 is a cross-sectional side elevation view of the resist system ofthe present invention in place on a conductor coated substrate;

FIG. 2 is a partial cross-sectional side elevation view illustrating thepractice of the present invention;

FIG. 3 is a cross-sectional side elevation view illustrating theimpingement of laser light on the resist system of the present inventionthrough a mask; and

FIG. 4 is a cross-sectional side elevation view similar to FIG. 3, butmore particularly illustrating the employment of vacuum means forremoving debris created by the ablation process.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates, in a cross-sectional side elevation view, a portionof a workpiece 20 processed in accordance with the present invention.For example, substrate 10 on which is disposed a conductive layer 12,may comprise a polymeric material which is particularly advantageous foruse as a substrate in a printed circuit board which is readily moldable.Substrate 10, which may, for example, comprise a material fabricatedfrom ULTEM™ polymeric resin, may also preferably contain filler materialsuch as glass fibers for added strength. Typically, the added glassfibers possess a length approximately 10 times their diameter. Theinclusion of such fillers, although desirable for strength, increasesthe surface roughness. It should also be noted that during the processof patterning conductive layer 12, which typically comprises a materialsuch as copper, it is highly desirably that substrate 10 not be damagedin the patterning of conductive layer 12 and that upperlayer 16,discussed more thoroughly below, not be adversely affected by theconductor etchant.

In certain circuit board fabrication processes, particularly thoseemploying fibrous filler material, palladium seeding is used. However,palladium material is prone to migration along fiber induced paths inthe polymer substrate. The palladium material can lead to the formationof undesirable conductive pathways between otherwise insulatedconductive patterns, as, for example, by electro-migration. However, itis possible to convert this palladium material to a non-conductivecompound. This process is compatible with the present invention and isdesirable when palladium seeding is employed, but forms no part of thepresent invention.

For the practice of the present invention, it is seen in FIG. 1 thatconductor coated substrate 10 includes a dual layer resistconfiguration. First or underlayer 14 comprises a thin layer ofablatable, photoabsorptive material. In particular, when the underlyingsubstrate comprises a polyetherimide such as ULTEM™ polymeric resin,underlayer 14 preferably comprises a polysulfone. This layer ispreferably between about 0.1 microns and about 10 microns in thickness.The resist system also includes thick upper layer 16 of substantiallytransparent material. In particular, in preferred embodiments of thepresent invention, it is desirable that upper layer 16 be transparent toradiation produced by excimer lasers in the ultraviolet range and thatunderlayer 14 exhibit absorptive properties in this frequency region sothat the underlayer is ablatable. It is also preferred that upper layer16 be between about 1 micron and about 100 microns in thickness. In thepractice of the present invention, underlayer 14 is ablated and upperlayer 16 is blown away during the ablation process. It is also desiredthat upper and lower layers 16 and 14 exhibit differential solubilitiesso that the upper and lower layers are not confused by the applicationof upper layer material to lower layer material.

FIG. 2 illustrates one method of practicing the present invention withrespect to the patterning of the resist system described herein. Moreparticularly, it is seen that a pulsed laser beam, as from an excimerlaser, is focused with a quartz beam focus lens 26 so as to produce beam30 passing through mask 24 and impinging upon workpiece 20 which isdisposed on X-Y positioning table 22. Although not visible at the scaleshown in FIG. 2, workpiece 20 (as is more particularly seen in FIG. 1)contains the dual resist system described herein.

While FIG. 2 shows a lens system for concentrating laser producedradiation, lens arrangements are not necessary for the practice of thepresent invention. With sufficiently powerful lasers, a concentrationmechanism is not required. However, such lasers are generally expensive.Accordingly, it is sometimes desirable to use the dual layer resistsystem of the present invention with concentrating optics; moreparticularly, it is possible to move the beam over the surface in aboustrophedonous manner to achieve the desired coverage. Thus, the X-Ypositioning table shown is optional, particularly if an optical beamscanning mechanism is employed. It is only important that the beam andworkpiece move relative to each other. Also, depending on the resolutiondesired for the conductive patterning, the mask itself is also optimal.

With respect to the mask itself, some special considerations should benoted particularly as a result of the high energy density present in theradiation beam. In particular, the mask preferably comprises a quartzsubstrate on which is disposed desired patterning in the form of areflective, as opposed to an absorptive metal. Desirable metals for thispurpose include aluminum and silver. Dielectric coating can bedeposited, but are difficult to apply and expensive. Aluminum masks havebeen employed and have been found to work. In one embodiment, a maskcomprised a quartz substrate about 100 mils in thickness. The quartzsubstrate included a patterned layer of titanium about 1,000 angstromsin thickness. The titanium facilitates adhesion of a reflective metallayer above it. Over the patterned titanium layer a layer of aluminumwas disposed which was about 25,000 angstroms thick. Such masks shouldbe employed with the patterning being disposed closer to the radiationsource than to the workpiece. Thus, any debris which happens to bedeposited on the mask can be wiped off easily without physicallycontacting the delicate pattern layer.

A mask which is particularly applicable for use in the present inventionmay be produced as follows: a quartz substrate is thoroughly cleansed ina mixture of 50% concentrated sulfuric acid and 50% hydrogen peroxide orother oxidizer. In the case of a mixture comprising 50% concentratedsulfuric acid and 50% hydrogen peroxide at a solution concentration ofapproximately 30% and immersion time of approximately 20 minutes hasbeen found to be satisfactory, during which time the solution fizzesactively and is particularly effective for removing organiccontaminants. The quartz substrate is then thoroughly rinsed in waterand processed through a standard VLSI plate washer. The cleansed andrinsed substrate is then disposed in a vacuum system at a temperature ofapproximately 50° C. for two hours. This time appears to be a minimumtime for satisfactory results. Next, a thin layer of titanium isdeposited on the quartz substrate, as for example, by electron beamevaporation. The titanium layer is preferably approximately 1,000angstroms thick. The titanium is found to be particularly desirable forimproved adhesion to the quartz material. Next, a reflective metal layeris deposited. Aluminum or silver is effective for this purpose. Forexample, in one embodiment, a layer of aluminum approximately 25,000angstroms thick was employed. The reflective metal and titanium layersare then patterned by standard photolithographic methods. In general,the reflective metal layer is preferably between about 20,000 and 25,000angstroms in thickness.

FIG. 3 provides a more detailed view of the operation of the presentinvention than the overall view shown in FIG. 2. FIG. 4 is similar toFIG. 3 except that it illustrates a vacuum system for removal of debris27 produced during ablation of layer 14 and the subsequent "blowingaway" of thicker layer 16. It is noted that the use of ablatablematerial 14 in a thick layer by itself is not adequate to solve theproblem of patterning three-dimensional parts as described herein. Moreparticularly, it is seen that the soot and debris produced by theablation of a thick layer of resist material would actually bedeleterious to the patterning process. However, it is noted that in theoperation of the present invention, upper layer 16 is not ablated, butrather is blown away, that is, it is lifted by gas formation resultingfrom the ablation of photoabsorptive layer 14. While the system ofdebris removal shown is a vacuum system, it is also possible, thoughless desirable, to blow the soot and debris away from the ablation site.While workable, this does not afford desirable levels of soot and debriscontrol. This is best achieved with a vacuum system. In particular, inexperiments carried out in support of the present invention, a speciallyfabricated vacuum nozzle was employed which partially surrounded theincident laser beam and was able to minimize debris problems.

In accordance with the present invention, printed circuit boardscomprising a polyetherimide such as ULTEM™ polymeric resin wereemployed. These boards were then palladium seeded and electrolesslyplated to provide a copper surface coating of approximately 25 micronsin thickness. As described above, "palladium kill" treatments areemployed to reduce shorting effects that can occur with palladiumseeding. Such printed circuit boards are then baked for adhesionpromotion and are now ready for resist deposition. However, tarnish onthe copper from the bake is preferably scrubbed off using a standardprinted circuit board cleaner method using Scrub Cleaner No. 11 (assupplied by the Shipley Company of Newton, Mass.) and a brush.

Such boards are then rinsed and dipped in methanol and spun dried andthen oven dried. The parts are then sprayed with a mixture ofpolysulfone and O-dichlorobenzene. A mixture of 2.5% UDEL™ 1700polysulfone (as supplied by the Amoco Chemical Company) was employed.The parts were then air dried at room temperature for approximately 5minutes and then placed in an oven at a temperature of 140° C. for 5minutes. The parts were then cooled to room temperature. Polysulfonesare inert with respect to methylmethacrylate materials such as KRYLON™(as sold by E. I. DuPont de Nemours Company) at least partially as aresult of their different solubilities with respect to each other. Theworkpieces are now ready for application of the upper, thicker layer ofthe resist system of the present invention. The upper layer may beapplied using a totally spray process or a combination of dip and sprayprocess steps to ensure complete coverage of plated through-holes, ifthey are present. The workpieces used in the present example were infact of the plated through-hole variety. A solution of 20% KRYLON™ byweight in methanol as a dip vat was employed. The KRYLON™ employed wasKRYLON™ Clear-150, a product of the Borden Company. The workpieces areimmersed and swished in the dip vat to ensure that no bubbles occur inthe through-holes. The workpiece is slowly withdrawn at a rate ofapproximately 10 inches per second for a three inch workpiece. Parts arethen set aside to allow even drying and are then placed on a rack fordrying at room temperature for approximately 5 minutes. The parts werethen oversprayed with another mixture of KRYLON™ Clear-150. It is to benoted that polymethylmethacrylate (PMMA) or polyethylmethacrylate (PEMA)or any other ultraviolet transmissive coating could be used as long asdifferential radiation transmission properties and solubilities aremaintained. Polymethylmethacrylate is a generic polymer available frommany sources including the Aldrich Chemical Company.

In one example carried out in accordance with the present invention, theKRYLON™ overspray layer comprised a mixture of 400 grams of KRYLON™ 150,175 grams of toluene, 125 grams of xylene and 50 drops of BE-173 (a flowcontrol agent supplied by the Nazdar Corporation of Chicago, Illinois).The workpieces were sprayed on both sides, dried at room temperature forapproximately 5 minutes, dried at a temperature of 50° C. for 5 minutesand then at a temperature of 90° for 10 minutes, then at a temperatureof 160° C. for 30 minutes and then finally cooled to room temperature.The KRYLON™ upper layer merely dries over the UDEL™ polysulfone layerwithout disturbing it because of their mutual solubilitycharacteristics.

The workpiece is now ready for ablation by a focused laser beam or by amore powerful unfocused laser beam. A focused excimer laser operating ata wavelength of 308 nanometers and employing unstable optics for highcollimation and minimum divergence was employed. A Questek 2860 laserwas used having a 1×2 centimeter rectangular beam at 180 millijoules ata pulse rate of 80 Hertz focused through a 4 meter quartz lens at 120inches to increase the power density desired for clean ablation. Thespot size was approximately 8.7 millimeters by 4.7 millimeters. An X-Yprogrammable positioning table that held the pattern mask in place wasemployed. The programmable positioning stage held the mask and theworkpiece. The vacuum particle remover was held stationary in closeproximity to the actual ablation site. Two passes were employed oversensitive three dimensional areas to ensure clear ablation. When theworkpiece is scanned, the excimer laser beam selectively ablates off allresist except where copper is desired (at least in one embodiment).

It is noted that the resist method of the present invention may beemployed in two different ways. In particular, thin conductive layer 12,approximately 1 to 3 microns in thickness, which is exposed by theablation process described herein may thereafter be built up selectivelyby electrodeposition processes after which the resist layers are removedand an etching step is performed to remove the thin conductive layer notexposed by the resist. This is a positive deposition system. This methodhas the advantage of producing conductive patterns exhibiting steepvertical walls.

However, it is also possible to employ the resist and method of thepresent invention in a negative deposition system in which the exposedcopper is etched, the resist material is removed and, if desired,electroless deposition is selectively carried out to build up theremaining conductive patterns. Copper etching and electrodepositionmethods for selective buildup are well known in the art. It is furthernoted though that metals such as nickel may also be employed asconductive layer 12.

When the conductive layer to be patterned comprises copper, ferricchloride is a desirable etchant. Thin, base layers of copper may beremoved in a ferric chloride bath by immersion for about 90 seconds at atemperature of about 40° C.

When the underlying substrate supporting the conductive layer comprisesa polyetherimide such as ULTEM™, it is important that materials used toremove the thick and thin polymer or resist layers be compatible withthe substrate. This is particularly true for the solvent used to removethe thick layer. For example, the ablated part is preferably dried forabout two minutes at a temperature of about 90° C. The part is thenpreferably washed in acetone for about five minutes and then in a halfand half mixture of cyclohexanone and xylene(s). This is preferablyfollowed by a one minutes wash in acetone after which an acetone sprayrinse is applied to the board to remove anything remaining of the twolevels. The board is then dried and ready for use, as a printed circuitboard, for example.

When the underlying substrate comprises relatively inert material suchas a ceramic, the range of solvents useful for removing a polysulfonelayer is increased. Suitable solvents for polysulfone in this situationinclude acetophenone, chloroform, cyclohexanone, chlorobenzene,dimethylforanide, dioxane, methylene chloride and tetrahydrofuran.Blends of solvents which are suitable for this purpose includetoluene/cyclohexanone, toluene/acetone, xylene/cyclohexanone andtoluene/acetone/cyclohexanone. See Table I below for the relevantsolvent weight ratios.

                  TABLE I                                                         ______________________________________                                                            Wt. Ratio                                                 Solvent Blend       of Solvents                                                                              % Solids                                       ______________________________________                                        Toluene/Cyclohexanone                                                                             50/50      20                                             Toluene/Cyclohexanone                                                                             75/25      20                                             Toluene/Acetone*    70/30      30                                             Toluene/Acetone/Cyclohexanone                                                                     65/25/10   20                                                                            25                                                                            30                                                                 35/15/50   20                                             Xylene/Cyclohexanone                                                                              50/50      20                                             ______________________________________                                         *Cannot be diluted below 30% solids                                      

From the above, it should be appreciated that the resist configurationand method of the present invention provide a significant advantage forpatterning three-dimensional parts. It is particularly seen that thesystem of the present invention provides a significant step forward inthe utilization of moldable plastic printed circuit boards, particularlythose containing structural and design elements such as channels, ridgesand bosses. It is further seen that the system of the present inventionprovides a process for the rapid patterning of high resolution lines ondesirable polymeric surfaces, even when such polymers contain fillmaterial which contribute to surface roughness. It is further seen thatthe system of the present invention meets all of the aforementionedobjects recited herein.

While the invention has been described in detail herein in accord withcertain preferred embodiments thereof, many modifications and changestherein may be effected by those skilled in the art. Accordingly, it isintended by the appended claims to cover all such modifications andchanges as fall within the true spirit and scope of the invention.

The invention claimed is:
 1. A two level resist configuration comprisinga thin underlayer of ablatable photoabsorptive polymer and a thick upperlayer of substantially transparent material overlying said polymer. 2.The configuration of claim 1 in which said underlayer is photoabsorptivein the ultraviolet range.
 3. The configuration of the claim 1 in whichsaid underlayer is between about 0.1 microns and about 10 microns inthickness.
 4. The configuration of claim 1 in which said upper layer isbetween about 1 and about 100 microns in thickness.
 5. The configurationof claim 1 in which said underlayer comprises material selected from thegroup consisting of polysulfones.
 6. The configuration of claim 1 inwhich said upper layer comprises material selected from the groupconsisting of polymethylmethacrylate, polyethylmethacrylate andpolycarbonates.
 7. The configuration of claim 1 in which said upperlayer is ultraviolet transmissive.
 8. The configuration of claim 1 inwhich said upper layer and said underlayer exhibit differentialsolubilities with respect to each other.
 9. The configuration of claim 1in which said underlayer is disposed in contact with a metal coatedsubstrate.
 10. The configuration of claim 9 in which said metalcomprises copper.
 11. The configuration of claim 9 in which saidsubstrate comprises a polymer.
 12. A method for photopatterningconductive patterns, said method comprising the steps of:disposing athin layer of ablatable photoabsorptive polymer on a substrate having aconductive layer thereon, so that said polymer material is in contactwith said conductive layer; disposing a thick layer of substantiallytransparent material over said polymer, said photoabsorptivity and saidtransparency being determined with respect to the same frequency range;and directing a beam of laser energy through said thick layer so as toablate said polymer layer and so as to simultaneously remove said thicklayer above said ablated polymer layer, whereby select portions of saidconductive layer are exposed through said thick and thin layers.
 13. Themethod of claim 12 in which said thin layer is selected from the groupconsisting of polysulfones.
 14. The method of claim 12 in which saidthick layer is selected from the group consisting ofpolymethylmethacrylate, polyethylmethacrylate and polycarbonates. 15.The method of claim 12 in which said laser energy is produced by anexcimer laser.
 16. The method of claim 12 in which said laser energy isin the ultraviolet frequency region.
 17. The method of claim 12 in whichsaid thin layer is between about 0.1 microns and about 10 microns inthickness.
 18. The method of claim 12 in which said thick layer isbetween approximately 1 and approximately 100 microns in thickness. 19.The method of claim 12 further including the step of etching saidexposed conductive layer.
 20. The method of claim 19 further includingthe step of removing said thick and thin layers.
 21. The method of claim12 further including the step of depositing additional conductivematerial on the exposed portion of said conductive layer.
 22. The methodof claim 21 further including the step of removing said thick and thinlayers.
 23. The method of claim 22 further including the step ofremoving a sufficient thickness of said conductive layer, so that theremaining pattern is substantially the same pattern as produced in saidthick and thin layers by said beam.
 24. The method of claim 12 in whichsaid conductive layer comprises copper.
 25. The method of claim 12 inwhich said beam is directed through a mask.
 26. The method of claim 25in which said mask is not in contact with said substrate.
 27. The methodof claim 12 in which said substrate exhibits a non-planar surface. 28.The method of claim 12 in which said substrate contains feed-throughapertures.
 29. The method of claim 12 further including removing saidablated material by vacuum means as it is produced.
 30. The method ofclaim 12 further including removing said ablated material by blowing itaway from the ablation side.
 31. The method of claim 12 in which saidbeam is focused.
 32. The method of claim 12 further including the stepsof:depositing additional conductive material on conductive materialexposed by said laser beam; removing said unexposed polymer and saidtransparent overlayer material; and removing sufficient conductormaterial so that conductor material is present only where said laserbeam was directed.
 33. A mask for use in high energy density laserpatterning, said mask comprising:a quartz substrate; a patternedtitanium layer disposed on said substrate; a patterned reflective metallayer disposed over said titanium layer, said reflective layercomprising material selected from the group consisting of aluminum and34. A method for making a mask for use in high energy density laserpatterning, said method comprising the steps of:cleansing a quartzsubstrate in a mixture of sulfuric acid and oxidizer; rinse and vacuumdry said cleansed substrate at an elevated temperature; deposit a layerof titanium on said substrate; deposit a layer of reflective metal oversaid titanium layer; and pattern said titanium and reflective metallayers.